Bug 1256530 - Fix compiler warnings about casting to larger size. r=hsivonen

Specifically, the Microsoft VS2015u1 compiler said:
warning C4312: 'type cast': conversion from 'unsigned int' to
'nsIContent *' of greater size

parser/html/nsHtml5Highlighter.cpp

@@ -648,7 +648,7 @@ nsHtml5Highlighter::AllocateContentHandle()
     mHandlesUsed = 0;
   }
 #ifdef DEBUG
-  mHandles[mHandlesUsed] = (nsIContent*)0xC0DEDBAD;
+  mHandles[mHandlesUsed] = reinterpret_cast<nsIContent*>(uintptr_t(0xC0DEDBAD));
 #endif
   return &mHandles[mHandlesUsed++];
 }